Frequently Asked Questions - 2008/08
By: Bob Aberle | [email protected]
New two-stroke glow-engine book
PLEASE WRITE in with your questions, since that is the only way we can keep this column format going. When referring to already published Q/As (for follow-ups), provide the number as a reference.
Notice that references to addresses and Web sites are now placed in a group, separate from the text, at the end of this column under "Sources."
Q358
"I still enjoy the sound and operation of two-stroke glow-fueled model aircraft engines. I've used many of these engines over the years with excellent success. I've read several books on the subject, but now feel I need something more than just the basics. Is there anything new out there (besides electric power)?"
A358 In the early 1990s, long-time model-engine expert and AMA Hall of Famer Dave Gierke wrote an excellent book titled 2-Stroke Glow Engines for R/C Aircraft: Volume 1. It has been one of the fueled-engine flier's standard references for many years.
Now a retired schoolteacher, Dave is devoting much more time to being a model-aircraft author. His latest book is exactly what you requested: Power: Beyond the Basics: 2-Stroke Glow Engines Vol. 2. The book is full color, published in an 8 1/2 x 11-inch format, and contains 143 pages of super-detailed current engine data and references.
The many chapters include such subjects as throttle design and development, how carburetors work, understanding fuel-delivery systems, future fuel-injection possibilities, fuel components, and choosing the right type of fuel for your engine.
Air Age Media publishes this book. It is available through RCStore.com, from the AMA, and directly from Dave. The list price is $24.95.
I may be into electric power for the most part, but this book provides some incredible information about two-stroke glow engines.
Also included in this column:
- Bungee-cord aircraft launching
- Calibrating small digital scales
- Biplane CG calculation
- Servo torque calculator and more!
- Animated engine operation
Q359
"I have a new electric-powered ducted-fan scale aircraft and want to be able to catapult launch it with the help of a bungee or elastic cord. Can you provide some of the details, like type of cord, length, attachment to the models, tension, etc.?"
A359 I've seen noted model author Jim Ryan of Cincinnati, Ohio, launch his various ducted-fan aircraft at the NEAT (Northeast Electric Aircraft Technology) Fair every year. I asked him your questions, and he replied:
"My bungee setup is pretty simple. I use a 20-foot length of a rubber tubing called 'Thera-Band' that can be purchased at Total Home Comforts. You want the blue color that denotes 'extra heavy duty.' A 25-foot spool will cost you $33.95.
"This elastic cord (bungee as we call it) is attached to a tent stake at one end and at the other end to 20 feet of glider towline. This towline isn't really critical; any lightweight nylon cord (or fishing line) should be fine. I take care about the links because that's where breaks occur. I use a 1-inch-diameter welded steel ring (from The Home Depot) between the stake and the bungee.
"On the other end (aircraft end) I took a short piece of dowel and drilled a hole down the center for the towline. The towline is then threaded through and knotted. I worked down inside the rubber tubing and then tied a ligature around the end. By doing this there are no strain points, so a break is very unlikely.
"Finally I have a smaller 3/4-inch-diameter welded steel ring at the end of the towline, and this gets hooked to the model. Standard towline hooks can be used on your model with the exact location resulting after a few trial launches.
"I've never fooled around with launching ramps, but some guys really like them. There are lots of resources on this subject on RCGroups and modelers who have done this in various ways. You should try a search on that forum.
"As far as the amount of tension placed on the bungee cord, the really scientific guys use a fish scale to measure the pull. I've heard that the rule of thumb is like 5 times the weight of the model.
"I tend to do it by feel: as you pull back on the bungee the tension will build steadily. Then there comes a point where tension increases rapidly. You're now close to the elastic limit of the cord, and this is the point to stop, and take one step forward from that point.
"I like this approach better than using a scale; the bungee only has so much to give, and if that's not enough, you need to double the bungee or use heavier cord. Just be very careful when doing all of this, because a bungee cord that gets loose can be outright dangerous."
Q360
"I build mostly micro/indoor models, which in many cases may only weigh a few grams. I've been concerned that my small-capacity digital scale might be out of calibration. Are there any sources for known weights that can check on the scale calibration?"
A360 This reader is lucky, because I received an interesting tip from another reader that will help answer this question. The latter informed me that all standard mint coins of the same denomination have close to the same weight. Typical weights are:
- Penny: 2.500 grams
- Nickel: 5.000 grams
- Dime: 2.268 grams
- Quarter: 5.670 grams
As shown, a nickel on my 120-gram maximum-capacity digital scale weighed exactly 5.0 grams. That means it is close enough. I don't believe these scales provide for adjustments, but using these coin standards should enable you to come up with a calibration conversion chart. This will allow you to zero in on exact weights.
Q361
"I'm trying to design my own biplane and was wondering how you calculate the center of gravity when two wings are involved?"
A361 Just to make sure, I asked my flying partner, Tom Hunt. Among many things, he is an aerodynamics expert.
He was kind enough to provide a descriptive diagram, which I have included in this column, that explains exactly how the biplane CG can be calculated. Thank you, Tom!
Q362
"I noticed that all RC servo manufacturers state the output torque for each unit. This torque figure is expressed usually in Ounce/Inches (Oz./In.). Knowing that this amount of power (torque) is available how do I relate it to the model speed, control surface size and the amount of deflection?"
A362 I did a considerable search on the Internet and came up with an excellent Web site identified as "Electric Flight in Colorado." I was unable to get the name of the person who is responsible for this wonderful site, but his e-mail address is on the home page if you want to thank him for this effort.
The first thing I found on this Web site was a Servo Torque Calculator. You enter your model's estimated maximum speed, control-surface width and length, and maximum deflection. Then the calculator gives you the required servo torque.
That by itself would prove to be interesting to many. But I went deeper into this site and found other useful items, such as a Wing Loading calculator, Glow-to-Electric Conversion calculator, and an Electric Motor calculator. This information is freely available to anyone who visits the site.
I've only begun to play with some of these features and found it to be totally fascinating. Please at least give it a try.
Q363
"I've always been fascinated with the operation of various engines, especially those used for model power applications. Is there any way I can learn about the operation of these engines in a simple format—not too technical?"
A363 Reader Joe Mielnicki referred me to a Web site that Matt Keveney created. It is an amazing find. Matt wrote:
"I have loved mechanical things since I was a kid. Engines in particular have always intrigued me. All my life I've pored over books, studying cutaway drawings, hungry to understand how things work. These pages are an attempt to share that magic."
As you work through Matt's site, you can find animated drawings that show the operation of engines such as four-strokes, two-strokes, Wankel, Gnome rotaries, and jet propulsion, CO2 motors, and many more. You owe it to yourself to go to this site and look through it in detail. You won't be disappointed.
MA
Sources
- Air Age Media
(203) 431-9000 https://secure.rcstore.com
- AMA store
(800) 435-9262 www.modelaircraft.org/shopama
- Dave Gierke
1276 Ransom Rd., Lancaster, NY 14086
- Total Home Comforts
(888) 358-0298 www.totalhomecomforts.com/tubing.htm
- Electric Flight in Colorado
www.csd.net/~cgadd/eflight/calcs_servo.htm
- Matt Keveney
Transcribed from original scans by AI. Minor OCR errors may remain.
